Conventionally, a combustion control in an optimum state is executed while always keeping a balance between an amount of the air-fuel mixture and an amount of the lead air, by interlocking and driving the lead air control valve shaft and the air-fuel mixture throttle valve shaft of the carburetor of the stratified scavenging two-cycle engine.
The stratified scavenging two-cycle engine is structured such that a lead air precedently having flown into a cylinder in a scavenging stroke is flown out to an exhaust port together with a combustion gas, and an air-fuel mixture flowing into the cylinder after the lead air can be stored within the cylinder. Accordingly, it is possible to prevent a so-called short circuiting phenomenon, that is, the air-fuel mixture flowing into the cylinder is discharged to an atmospheric air from the exhaust port together with the combustion gas, it is possible to widely reduce an exhaust gas concentration, and it is possible to reduce a dissipation of a specific fuel consumption.
A transmissible connecting mechanism is used as a control mechanism which can obtain an optimum opening degree of the lead air control valve with respect to an opening degree of the throttle valve in the carburetor, for controlling a timing at which the lead air and the air-fuel mixture are flown into the cylinder, an inflow amount and the like. As a transmissible connection between a lead air control valve shaft and an air-fuel mixture throttle valve shaft of a carburetor in which both the valve shafts are arranged in a parallel state, for example, there has been proposed a diaphragm carburetor using a cam mechanism and a link mechanism, by Japanese Patent Application Laid-Open (JP-A) No. 2000-314350 (patent document 1).
The diaphragm carburetor described in JP-A No. 2000-314350 is provided with a structure shown in FIG. 8. In other words, an operation lever 66 is borne in one end of a throttle valve shaft 63 of a throttle valve (not shown) arranged within a carburetor casing 60. The operation lever 66 is arranged in one end 67 of the throttle valve shaft 63 so as to be relatively non-rotatable, and is elastically energized in a valve closing direction of the throttle valve 62 via a restoring spring 68. Further, the operation lever 66 is connected to a carburetor control cable or a similar structure thereto in accordance with a non-illustrated aspect, and can regulate an opening degree of a throttle valve (not shown) arranged within the carburetor casing 60.
A lever 69 is borne to the other end 67′ of the throttle valve shaft 63 so as to be relatively non-rotatable as shown in FIG. 9. In the same manner, a lever 71 is borne to an end portion 70 of a shaft 65 of the lead air control valve. The throttle valve shaft 63 and the shaft 65 of the leading air control valve are arranged in parallel, and the levers 69 and 71 are connected to each other via the drawbar 72. One end of the drawbar 72 is rotatably engaged with the lever 71, and the other end is arranged within a vertical slit 74 provided in a lever 69 so as to extend approximately in a rotational direction 73. Accordingly, a link mechanism serving as a transmissible connecting portion 76 is structured by the levers 69 and 71 and the drawbar 72.
The transmissible connecting portion 76 formed between the shaft 65 of the lead air control valve and the throttle valve 63 is driven by a rotation of the throttle valve shaft 63, and the connection between the lead air control valve and the throttle valve 62 of the carburetor is achieved dependently on a position. As shown in FIG. 12, the restoring spring 68 is applied to the throttle valve shaft 63 in a valve closing direction of the throttle valve 62, and a coil spring 75 is correspondingly to the shaft 65 of the lead air control valve. The coil spring 75 determines a valve closing position of a butterfly-shaped throttle valve structured as the lead air control valve. As shown in FIG. 13, home positions of the throttle valve shaft 63 and the shaft 65 of the lead air control valve can be respectively determined by the restoring spring 68 and the coil spring 75.
As a structure of the cam mechanism serving as the transmissible connecting portion 76, as shown in FIG. 10, the levers 69′ and 71′ having the cam profile portion 80 and the cam profile portion 81 are respectively attached to the throttle valve shaft 63 and the shaft 65 arranged in parallel. In the case that the throttle valve 63 is moved against a force of the restoring spring 68 in the valve opening direction 73 together with the throttle valve within the carburetor, the shaft 65 of the lead air control valve is structured such as not to be operated at a time of an idling and in an idling lower range until an idling path portion 77 between a free end 79 of the lever 69′ and a free end 78 of the lever 71′ is overcome.
When the cam profile portion 80 of the free end 79 is brought into contact with the cam profile portion 81 of the free end 78, the throttle valve 62 within a intake pipe line 61 already exists at a partial load position. If the throttle valve 62 is further opened at this time point, the shaft 65 of the lead air control valve is taken in the valve opening direction 73, and a regulating distance at that time can be determined by the cam profile portions 80 and 81 of the vertical edges of the levers 69′ and 71′.
Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-314350